As the formation became visible above New Zealand’s South Island, the country’s stargazing and astronomy social media groups were filled with photographs and theories about what might have caused the phenomenon.

While speculation about what had caused the clouds ranged from black holes to “a giant alien spaceship,” the prevailing theory is that the spiral was the result of vapors from a SpaceX rocket launch.

In the early hours of Sunday morning, SpaceX’s Falcon 9 rocket launched the Globalstar FM15 satellite into low-earth orbit from Cape Canaveral Space Force Station in Florida.

Alasdair Burns, who runs stargazing tour company Twinkle Dark Sky Tours on Stewart Island off New Zealand’s southern coast, was alerted to the cloud formation by a text message at around 7:27 p.m. local time on Sunday.

Seeing the cloud left him “completely awestruck,” he told Fortune on Tuesday.

“Not much visibly changes in the night sky in short time frames, so to see what looked like a spiral galaxy suddenly overhead was unsettling,” he said.

Burns said he immediately thought the clouds were likely caused by a rocket, having seen images of similar phenomena that had occurred in the aftermath of a launch.

“Even knowing that it was still eerie to watch,” he added. “I’m quite certain that it was a rocket, whether it was that particular SpaceX rocket or not. There was a similar phenomenon in Norway in 2009 which turned out to be a Russian missile test, and the rocket propellant created a spiral very similar in appearance to the one on Sunday.”

The New Plymouth Astronomical Society, based in northern New Zealand, said in a statement on Facebook that the so-called spiral was “most likely a ‘fuel dump’ or ‘exhaust plume’ from a SpaceX rocket launch.”

“Similar effects have been seen before, and SpaceX’s Globalstar 2 FM15 was likely to have passed New Zealand around that time,” the organization said.

“It is shown in [an online video] passing South of NZ just over an hour into its flight, which would have been around 5:30 [p.m.] and would probably have passed again around 90 to 120 minutes later which would then have been around 7:30 [p.m.].”

Professor Richard Easther, a physicist at Auckland University, told the Guardian that the incident was “weird but easily explained,” noting that when propellant was ejected from the back of a rocket, water and carbon dioxide sometimes briefly formed a cloud in space that was illuminated by the sun.  

“The geometry of the satellite’s orbit and also the way that we’re sitting relative to the sun—that combination of things was just right to produce these completely wacky-looking clouds,” he said.

A spokesperson for SpaceX was not immediately available for comment when contacted by Fortune.